993 Head studs, two types?

[Henry Schmidt]

Originally Posted by ivath

From an earlier thread on this topic:





As an mechanical engineer I am very interested in an explination of the modern engineering practices that is used.
Maybe the Supertec stud is an ok product, but I get worried when you say the best available solution from Porsche is horrible and yours is "perfect".

The benefits of the Supertec studs and their design has been discussed ad nauseam.
Your attempt to characterize my statements as a claim of "perfection" strains credulity.
The Supertec stud is an option that has been installed without failure in hundreds of engines without complaint.
Use them or don't, it's your choice.

[porterdog]

What about toughness and yield strength (not elastic modulus as previously typed; whoops!)? It would seem like significant amount of the loading on a Porsche head stud could be wildly oscillating loads caused by the cantilevered mass of the jug/head/cam/tower responding to road imperfections whizzing by sideways at frightening speed....

In Henry's picture (post #25) are those heads intake port up?

[Walt Fricke]

No, have to be exhausts. Notice that the valves which are up look smaller in diameter, and the other ones have a dish in them, as only intakes do. Plus this motor must still have been running exhaust air injection, with the injectors in place with colored plastic caps.

[Weissach911]

Originally Posted by porterdog

What about toughness and yield strength (not elastic modulus as previously typed; whoops!)? It would seem like significant amount of the loading on a Porsche head stud could be wildly oscillating loads caused by the cantilevered mass of the jug/head/cam/tower responding to road imperfections whizzing by sideways at frightening speed....

In Henry's picture (post #25) are those heads intake port up?

The yield strength of this type of stud shouldn't really be an issue.

If the stresses produced by the applied preload and the expansion of the head and the casing caused the studs to yield then there would be obvious signs and virtually every engine would fail.

Precipitation Hardened Austenitic Stainless Steels would have a typical 0.2% Proof Stress of around 120ksi at room temperature and this would probably only reduce by 2% at 400 degF

The Fracture Toughness of these alloys are exceptional and the KIC is around
170MPa√m


17-4PH will almost certainly have a higher yield strength or around 140 ksi, the KIC will still be very good but is a lower at around 100MPa√m

The elastic modulus is important as it determines the increase in the preload caused by the thermal expansion of the cylinder/head.

As both Dilavar and 17-4PH have a Modulus within 1% there won't, however be much of a difference due to materials.

I have to say that I just don't believe that the effects of gyroscopic procession will have any influence on bolting

[Cloggie]

Not trying to be argumentative, but just a question for Henry. The heads that you pictured with the leakage, is it fair to assume that any time that you have removed heads that were retained by your studs that there was no leakage of that type?

Just wondering if the characteristic of a high performance head, balanced on an individual cylinder, retained to an aluminium case by 4 studs might have a degree of leakage regardless of the stud type? I guess I get suspicious about transient conditions such as very cold starts or such.

Steve, also curious from your side as you seem to inhabit the Dilavar perspective....has Henry's experience been borne out in your work as well?

D.

[Bill Verburg]

Bruce Anderson put this together a few years ago



There are indeed at least 2 different 993 head studs
993.101.172.02 used on all n/s 993 except RS
993.101.170.51 used on all turbos and RS

[Steve@Rennsport]

Originally Posted by Cloggie

Steve, also curious from your side as you seem to inhabit the Dilavar perspective....has Henry's experience been borne out in your work as well?
D.

JMHO,

Leakage that I see has been due to other things and it depends a lot on the engine in question: 3.6's and larger are more prone due to the length of the span between the upper & lower row of studs.
Those heads are susceptible to bending along that span so we employ several measures to deal with that issue.

We use the 993TT/RSR Dilavars in all of the aluminum-cased motors since that's what we've had the most success with. I've not seen a broken one to date, even in 13.5:1 race engines.

[Turbo_pro]

Originally Posted by Steve@Rennsport

JMHO,

Leakage that I see has been due to other things and it depends a lot on the engine in question: 3.6's and larger are more prone due to the length of the span between the upper & lower row of studs.
Those heads are susceptible to bending along that span so we employ several measures to deal with that issue.

We use the 993TT/RSR Dilavars in all of the aluminum-cased motors since that's what we've had the most success with. I've not seen a broken one to date, even in 13.5:1 race engines.

The heads Henry posted look to me to be 930 heads not 3.6 so head deformation is not the issue. In an earlier post, it says the heads are from a street driven low mileage 930.
I have seen broken 993TT studs in my limited exposure to the Porsche engine building world so I am surprised to hear Steve make this claim. I think I've even seen broken 993TT studs posted on Pelican forums.

[Steve@Rennsport]

Originally Posted by Turbo_pro

I have seen broken 993TT studs in my limited exposure to the Porsche engine building world so I am surprised to hear Steve make this claim. I think I've even seen broken 993TT studs posted on Pelican forums.

Hi Jim,

I didn't say that they never break; I simply wrote that I've not experienced that, as of yet,....

Ultimately, we are all simply the collective result of our own experiences and that is what makes us individuals. Its what makes life both interesting and fun,....

[Weissach911]

Originally Posted by Bill Verburg

Bruce Anderson put this together a few years ago



There are indeed at least 2 different 993 head studs
993.101.172.02 used on all n/s 993 except RS
993.101.170.51 used on all turbos and RS

I am surprised by the low coefficient of expansion shown for both the the Dilavar Studs and the Nikasil Cylinder.

Most Aluminium Alloys have a coefficient of expansion between 20 and 25ppm/degC and 14-15 is very low.

Even NASA398 which has been specirfically developed for Low Expansion is 18.4ppm/degC.

I am reasonably confident that Dilavar is 18ppm/degC.

[Turbo_pro]

Originally Posted by Steve@Rennsport

Hi Jim,

I didn't say that they never break; I simply wrote that I've not experienced that, as of yet,....

Ultimately, we are all simply the collective result of our own experiences and that is what makes us individuals. Its what makes life both interesting and fun,....

Wow, thank you for your cordial response. Hello to you. Life is fun and individual expression adds a great deal. It was not my intention to put words in your mouth but merely clarify your statement.
Since you're stating that you don't see 993TT studs breaking, I guess my next question is "are you currently building engines ?"

[blue72s]

Originally Posted by Steve@Rennsport

We use the 993TT/RSR Dilavars in all of the aluminum-cased motors since that's what we've had the most success with.

What about mag-cased motors (w/Nikasil cyls)?

[Steve@Rennsport]

Originally Posted by Turbo_pro

Wow, thank you for your cordial response. Hello to you. Life is fun and individual expression adds a great deal. It was not my intention to put words in your mouth but merely clarify your statement.

I'm glad you did,.....

Since you're stating that you don't see 993TT studs breaking, I guess my next question is "are you currently building engines ?"

Yessir,..been doing same for 35+ years.

[aws]

Weissach911[QUOTE]It is very unlikely that peak cylinder pressures cause the head to lift if it had been correctly fitted as this would have a very damaging influence on performance and reliability.

If the stud, however, has too much preload and peak cylinder pressures are too high then there is the likelihood that it can and will pull [QUOTE]

My experience has been that on a twin turbo 2.5 built on a early aluminum case, dilivar studs allowed the heads to blow off the cylinders far enough to blow the CE head gasket out of the groove and pinch it between the cylinder and the head. That problem was solved by installing steel studs and ni-resisting the cylinders and heads. The threads in the aluminum case were undamaged so my assumption at the time was that the studs were allowing the heads to lift at maximum cylinder pressure. My assumption on pulled steel heads studs on Mag case engines was the expansion rate causing excess torque due to thermal expansion and inability of the magnesium to deal with that increased torque. We have never had problems with the aluminum cases pulling head studs even at increased operating temperatures. After refreshing numerous racing engines and never having any head sealing problems, I have never considered anything but steel studs---and in the end that is what counts--experience---NOT theory.
aws
PS: Henry you have shown remarkable restraint with the engineering types.

[Weissach911]

[QUOTE=aws;6445636]Weissach911

dilivar studs allowed the heads to blow off the cylinders far enough to blow the CE head gasket out of the groove and pinch it between the cylinder and the head.

What I don't understand about this description is why a steel stud will survive when a Dilavar stud will allow this problem to occur.

Both studs would have a similar yield strength (Within 20%) and a similar preload so why does one stud fail so badly and the other survive so well?

If I understand correctly to allow the CE ring to escape from the groove the head would need to lift by several millimetres.

If we assume that 4mm would be a 'good' value then the stud would need to elongate by significantly more than its yield point and therefore the stresses would need to be in excess of 120ksi for the Dilavar stud.

A peak cylinder pressure of 2000psi would generate a force of about 62ksi in each stud. If we add this to the existing preload of around 50ksi then I would agree that this is far too close for comfort. Add a bit more load for expansion and you could well be in trouble.

The steel stud, assumimg you are using a higher strength material will see exactly the same loadings but the additional stress casued by differences in expansion will also result in a situation where the studs are very, very close to yield.

Add a detonation to the equation and I could well understand problems occuring.

It is also interesting that Porsche EB welded the heads onto some of their Turbo race engines but I am not sure how this would reduce stud loading.

If we now consider the 4mm displacement needed to allow the ring out of the groove then the stud would have to extend by this amount.

The yield extension of both steel and Dilavar studs will be about 0.3mm so the studs would have to stretch significantly.

Peak cylinder pressure has a duration of about 25 degrees of crank angle and at say 6000rpm then the peak force is maintained for about 42 milliseconds

If the material had to extend by 4mm during this time the 'impact rate' would be about around 7 metres per second.

I don't know of any steels that will transmit plastic waves at this velocity and the only conclusion is that the studs would have to snap.

I am afraid to say I find that unless explanations of problems of this type are backed up by some engineering justification they are just too big a leap of faith.

I am sad to say that the 'argument' about practical experience versus theory has been going on since the start of Scientific Experiment.

All of the real engineering progress we have made in the last two hundred years has been as a reult of improved understanding and better mathematical modelling so please lets not throw it all away over a few studs.

I think it is always important to try to justify claims that one product is good and another is bad and i have no vested interest in either type of stud.

[ivath]

[QUOTE=Weissach911;6445875]

Originally Posted by aws

Weissach911
I am sad to say that the 'argument' about practical experience versus theory has been going on since the start of Scientific Experiment.

All of the real engineering progress we have made in the last two hundred years has been as a reult of improved understanding and better mathematical modelling so please lets not throw it all away over a few studs.

I think it is always important to try to justify claims that one product is good and another is bad and i have no vested interest in either type of stud.

I agree with you. I am happy to see that somebody really wants to understand the problem.

[Henry Schmidt]

Although understanding the problem from a engineering point of view is a noble endeavor, in the real world we fix the problem not the blame. What we seek is a solution. Heads move, how can we make them more stable?
Over the years there have been a myriad of attempts to remedy the head to cylinder connection in the air cooled Porsche engine. Even Porsche has danced around the problem with many different "solutions" all from their engineering department all with limited success.
We can only control certain aspects of the process.
Replacing heads is expensive but doable (Bill @ extreme Cylinder Head) , although cylinders come in a plethora of materials, most function pretty similarly and the case is what it is. We can control heat to certain extent but basically we're stuck with a specific design. The only reasonable variable is head studs. We used the factory studs for years and weren't satisfied. We along with countless others, sought a more stable platform.
After seven different stud configurations we came up with a stud that performs better under most situations than the all thread Dilavar. Notice I say "most situations" because the product is still exposed to an imperfect environment. I use the word "performs" because we and many other engine builders have personal experience with the product and can see the results. Don't believe it, don't use it.
But to claim that the product does not perform better without ever trying it or because you can't explain why it works better seems just a touch squirrelly.
Break out your calculators and soldier on with your projects with my best wishes.

Merry Christmas form the guys at Supertec.

[Weissach911]

Originally Posted by Henry Schmidt

in the real world we fix the problem not the blame

Henry,

I believe that the basis of Ivath's original question was only to ask for some basic data and modelling to augment the comments which merely say Stud A - Good, Stud B - Bad. This type of statement relies on faith and it is hard for some of us to accept.

I am quite sure your studs work reasonably well and have never criticised them and do understand the thinking behind the material selection but in the engineering world I have always lived in engineering problems have first had to be researched, documented and analysed. Solutions have had to be justified in terms of data and some basic model to show how the 'fix' resolves the issues and their causes has had to be provided. Unless this work has been done 'fixes' can never be signed off and work has to continue until a solution is found.

As a Chartered Engineer I have worked all my life in the design and development of Power Station plant including steam boilers, pressure vessels and large (650MW) Steam Turbines and to suggest that I don't live in the real world is a bit of a kick in the teeth but maybe I do lack practical experience of high temperature machinery and materials

Have an excellent Christmas and a Happy New Year.

[BURN-BROS]

Are the numbers right on the spreadsheet?


If they are, the Dilivars result in a net LOSS of clamping force @ temp.....that can't be right....or is it?

[Flieger]

This is the most astute and nuanced/implicit argument I have ever read.